This invention relates to high-surface area anatase titania alkaline sol compositions useful as catalyst supports and binders, and methods of their manufacture. High surface area, or ultrafine, anatase titania (TiO2) is commonly used as a catalyst support material for reacting with atmospheric pollutants such as oxides of nitrogen, particularly from diesel engine exhaust, via reduction with ammonia or urea, in a process termed selective catalytic reduction (SCR). In this catalytic process, the titania is typically used as the support material for the active catalytic metal or oxide, which is typically vanadia or other active materials such as iron, cerium, copper, and/or manganese oxides. Anatase titania is also active by itself in the light-catalyzed (photocatalysis, PC) destruction of such atmospheric pollutants such as the oxides of nitrogen, sulfur, ozone, toxic and unpleasant odors such as VOCs, and particulate materials such as dust and dirt. The titania can be used alone, or can be mixed with other materials, and disposed as a coating on a surface. The titania coating, when illuminated by UV light, absorbs the UV light thereby driving the photocatalytic process which degrades, reduces, or oxidizes the pollutants. The titania may be provided as a stable, aqueous colloidal dispersion (a sol), that is, a mixture in which the titania particles are small enough to resist sedimentation over time. Examples of ultrafine anatase titania sols include S5-300A® and S5-300B®, which are peptized with acid and base, respectively, and are available from Millennium Chemical Co. Sol S5-300B® for example comprises titania in a weight % of 17.5±2.5, at a pH of 11.5±1, and has a surface area of >250 m2/g of dried product as measured by BET. In addition to serving as a catalyst material, small particles of anatase titania provided in the stable sol can be used as a binder material in order to improve the adhesion of other titania particles onto a monolith support. Further, the ability to provide titania catalytic materials and support materials in small particle form is particularly advantageous for adding SCR activity to a diesel particulate filter (DPF) by coating the pores in the walls of a wall-flow particulate filter. Such a combined particulate filter/SCR catalyst is termed SCR-F, and this approach offers significant advantages over separate SCR/DPF catalysts.
While S5-300B® titania sol has shown great usefulness as a titania source for use in production of catalytic materials, it has several short comings. S5-300B® has, as noted above, a titania content of about 17.5 wt %. It is desirable to increase the solids content of the sol for several reasons. First, a sol provided at higher solids will have lower freight and duty costs. Second, when used in a production process such as wash-coating a monolith, a higher solids sol will enable more of the titania solids to be deposited in one wash-coating step, which can lead either to improved functionality, lower processing costs, or both. Further, S5-300B® is stabilized at a pH of about 11.5 by the organic dispersant diethylamine (DEA), which is both strongly alkaline and miscible in water, and as such is a good alkaline peptizing agent However, this conventionally-available S5-300B® sol has a relatively high flammability (flashpoint 35° C.) due to the high vapor pressure and low boiling point of diethylamine which constitutes about 2.6 wt % of the sol.
While S5-300B® has the undesirable properties of being provided at relatively low solids content with a high vapor pressure dispersant, it does have the desirable properties of low viscosity and low surface tension. These desirable properties are useful in that they facilitate the ingress of the sol into the channels and/or pores of a monolith support, so that the washcoating process is improved.
It is thus desirable to develop an improved sol, which is provided at higher solids, with a lower vapor pressure, that can be made under relatively mild conditions, while still maintaining the favorable properties of low viscosity and low surface tension.
The peptization route to prepare stable titania sols is disclosed in U.S. Pat. No. 5,049,309, and more recently in US 2009/0062111 A1. In this approach, a precipitated hydrous titania precursor from the sulfate process can be used. The physical structure of this precipitated hydrous titania precursor is described in two references: Sathyamoorthy, S., et al, in Crystal Growth and Design, (2001) Vol. 1, No. 2, 123-129, and Jalava, J.-P., in Industrial & Engineering Chemistry Research, (2000), Vol. 39, No. 2, 349-361. To briefly summarize, such a precipitated material is comprised of small anatase primary crystallites, typically on the order of a few nm in size. These crystallites are further bonded together to form what are often referred to as primary aggregates which typically range in diameter from 50-100 nm. These primary aggregates are also further bonded together to form agglomerates that are roughly one or two microns (1 μm-2 μm) in diameter. The final agglomerated particle thus has an internal porous network. It is believed that in the peptization process, severe conditions of time, temperature and pH are used to create chemical forces that disrupt the forces that bond the primary aggregates together to form the micron-sized agglomerate. When these latter forces are overcome, the agglomerates are broken down into the primary aggregates roughly 50-100 nm in size. Under more severe peptization conditions, the primary aggregates can then be broken down into the primary crystallites. One object of the present invention is to provide an alternative means of breaking the agglomerates down into smaller particles that can be done under less severe conditions of pH, time and temperature, while enabling sols with higher solids content to be obtained.
For use of the titania sols as active catalyst materials, catalyst supports or catalyst binders, it is disadvantageous to use the hydroxides of Group IA or IIA elements as dispersants or peptizing agents because such alkali (such as NaOH and KOH) are strong catalyst poisons for SCR reactions, for example. Therefore, the alkaline dispersants of the present invention are restricted to organic bases (and hence can be burned off during the production process of the final catalytic article for applications such as SCR). Examples of weak organic bases such as NH3 and the alkanolamines, which have lower flammability and cost than DEA, are not as strongly alkaline as DEA, and hence do not effectively to peptize the titania to prepare a stable sol. An example of a very strong base that has lower flammability than DEA is tetramethylammonium hydroxide (TMAOH). This reagent, by virtue of the fact that it is a salt in aqueous solution, however, produces sols that have relatively high surface tension. Further, TMAOH and its decomposition products (amines) have very strong and offensive odors. Finally, TMAOH is a relatively expensive reagent compared to other organic bases such as the alkanolamines.
A titania sol which optimizes the optimal features of high solids content and stability and minimizes the features of flammability, viscosity and surface tension would be highly desirable.